On-board vehicle diagnostic of an oxidation catalyst

ABSTRACT

There is presented a method for monitoring an oxidation catalyst in an exhaust line of an internal combustion engine, wherein a catalyst diagnostic event comprises a test cycle during which a conversion capability of the oxidation catalyst is determined based on the exotherm generated by post-injection of fuel. The diagnostic event may only be initiated when the temperature of the oxidation catalyst lies within a predetermined temperature range.

FIELD OF THE INVENTION

The present invention generally relates to on-board diagnostics, andmore particularly to a method for monitoring the performance of anoxidation catalyst in the exhaust-line of an internal combustion engine.

BACKGROUND OF THE INVENTION

Catalytic converters are now conventionally featured on exhaust systemsof automotive vehicles to purify the engine exhaust gases. In dieselengines for example, the most commonly used catalytic converter is thediesel oxidation catalyst (DOC), typically associated with a DieselParticulate Filter (DPF). Its main function is to convert hydrocarbons(HC) in the exhaust gas stream to water (H₂O) and carbon dioxide (CO₂).These converters often reach 90% effectiveness, virtually eliminatingdiesel odor and helping to reduce visible particulates (soot).

The need for catalytic converters, especially on motor vehicles,increases with the increasingly stringent environmental protectionregulations in many countries. Also, with the upcoming EURO V and VIregulations, the on-board diagnostic of DOCs becomes mandatory for alarge number of vehicles. Such diagnostic consists in determining theperformance, i.e. conversion efficiency, of the catalyst to detectwhether the catalyst is still able to purify the exhaust gas accordingto the regulations, or whether it is too old/damaged and should bereplaced.

Indeed, over the long term, in other words over an average mileage onthe order of 100 000 km or an average operating time on the order of 1000 hours in the case of a motor vehicle engine, the capacity of acatalytic converter to convert the pollutant decreases steadily. Thatprocess is called “ageing” and it is caused by manifold physical andchemical environmental factors, which are unavoidable no matter howcarefully the catalytic converter is handled. The process of ageingdepends to a great extend on the strain to which the catalytic converteris exposed in its typical operation. Another phenomenon that may alterthe conversion efficiency of a catalytic converter is “poisoning”, e.g.if a fuel of poor quality is used.

Due to such usage-specific ageing of catalytic converters, it isvirtually impossible to make general predictions regarding thedurability of a catalytic converter. Further, general guidelines, ifany, on the length of time in use or the mileage after which a catalyticconverter would have to be replaced, would have to be especiallystringent to in order to prevent inactive catalytic converters fromcontinuing to be used. But this would obviously imply replacing, for thesake of prevention, catalytic converters that are still significantlyuseable.

The remedy for this situation is thus the monitoring of the functionalcapability of each catalytic converter that is carried out over the longterm and is reliable.

In diesel engines, on-board diagnostics of the DOC are typically basedon the exotherm generated by a post-injection (i.e. the injection offuel during the exhaust stroke). The post-injection increases the amountof uncombusted HC in the exhaust stream, which will be converted in theDOC into water and CO₂. This conversion involves exothermic oxidationreactions that produce an amount of heat that is directly linked to thepre-DOC concentration (i.e. the amount of HC entering the DOC). As it isknown to those skilled in the art, it is this exotherm generation thatis used in the regeneration of diesel particulate filters (DPF) toincrease the temperature of the exhaust gases in order to burn theaccumulated soot.

Accordingly, the DOC diagnostic in a diesel engine is typicallyperformed during the regeneration mode of the DPF, which implies acombustion mode with post-injection. The typical conversion efficiencydiagnostic consists in monitoring the exotherm of the DOC by comparingthe pre- and post-DOC temperatures of the exhaust gases, and thus checkwhether a post-injection actually causes an increase in theexhaust-gases downstream of the DOC. Performing the diagnostic during aDPF regeneration mode also ensures that the DOC is at a relatively hightemperature, since a minimum temperature of the DOC is required forenabling the conversion.

OBJECT OF THE INVENTION

The object of the present invention is to provide an alternative methodof monitoring an oxidation catalyst that permits a reliable, long termmonitoring of the catalyst conversion efficiency.

SUMMARY OF THE INVENTION

The present invention is based on the observation that when thediagnostic of a catalytic converter is performed in a high temperaturerange thereof, as is typically the case during a regeneration event,there is a risk of erroneous assessment. The reason for this is that an(highly) aged catalytic converter can still have a good efficiency inthe high temperature range and be inefficient in a colder temperaturerange of an emission test cycle. Therefore, a regeneration event with ahigh DOC temperature and/or a high temperature of engine-out exhaustgases cannot be used to safely diagnose a DOC.

To overcome these drawbacks, the present invention proposes a method asclaimed in claim 1.

The present invention relates to a method for monitoring an oxidationcatalyst in an exhaust line of an internal combustion engine, wherein acatalyst diagnostic event comprises a test cycle during which aconversion capability of the oxidation catalyst is determined based onthe exotherm generated by post-injection of fuel. According to animportant aspect of the invention, the diagnostic event may only beinitiated when the temperature of the oxidation catalyst lies within apredetermined temperature range.

Furthermore, the test cycle comprises a post-injection event and anexotherm monitoring period that lasts, after the post-injection event,until essentially all of the heat stored in the oxidation catalyst hasbeen evacuated.

The predetermined temperature range is advantageously selected to beable to discriminate between a new or mildly aged catalyst and an agedcatalyst, and should thus cover a temperature range for which it isexpected that the conversion efficiency of a highly aged catalyst willbe sensibly below that of a more recent, functional catalyst (i.e.operating satisfactorily).

An alert signal may then be triggered when the conversion efficiencydrops below a conversion efficiency alert threshold.

In one embodiment, the predetermined temperature range is selected tocorrespond to a transition zone of the conversion efficiency for areference oxidation catalyst with given ageing. In other words, thepredetermined temperature range is preferably selected to correspond toa temperature range where the conversion efficiency of the referenceoxidation catalyst with a selected ageing is less than 100%, and morepreferably has a conversion efficiency sensibly lower than that of aless aged, functional oxidation catalyst. As it will be understood, thevalues of conversion efficiency values at different temperatures for thereference oxidation catalyst are calibrated and determined byexperimentation.

More preferably, the upper temperature limit of the predeterminedtemperature range corresponds to a conversion efficiency of no more than90% for the aged, reference oxidation catalyst.

In a preferred embodiment, the predetermined temperature range isselected so that its lower and upper temperature limits correspond to aconversion efficiency of about 40 to 80%, respectively, for an aged,reference oxidation catalyst, more preferably 40 to 60%.

Preferably, a further criterion for the lower temperature limit of thepredetermined temperature range is that it shall at least correspond toa predetermined conversion efficiency of a functional oxidation catalyst(with given ageing), which is preferably of at least 50%.

As it will be understood, the present diagnostic event is advantageouslyperformed when the engine is not operated in a DPF regeneration mode.

For increased accuracy of the diagnostic event, the latter is cancelledin case the temperature of the oxidation catalyst exits thepredetermined temperature range. This in particular if it happens duringthe post-injection event.

Also for enhanced accuracy, the oxidation catalyst temperature may bemonitored by means of a multi-slice model, and the diagnostic event mayonly be triggered and/or performed when the respective temperatures ofall of said slices lie within the predetermined temperature range.

Further, during the diagnostic cycle the engine parameters mayadvantageously be set to minimize the amount of HC in the engine-outexhaust gases, except for the post injection.

According to one embodiment, the measured exotherm is determined bymonitoring the temperature of exhaust gases exiting the oxidationcatalyst for a given time period after a post-injection.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1: is a graph showing the effects of ageing on catalyst efficiency;

FIG. 2: is a graph showing the measured DOC-out exhaust gas temperature(T_(out)) and the model exhaust gas temperature without post-injection(T_(out-mod)).

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present method will now be explained with respect to a preferredembodiment applied to the field of diesel oxidation catalysts (DOC).

As it is known to those skilled in the art, DOCs are now commonly foundin the exhaust system of diesel motor vehicles. They are often arrangedupstream of a diesel particulate filter (DPF) for assisting in theregeneration thereof. Indeed, in order to eliminate soot particlescollected in a DPF, it is conventional to increase the temperature ofthe exhaust gases by operating the engine in an adapted combustion modeinvolving (1) split and retarded fuel injections; and (2) performingpost-injection to increase the amount of unburned HC in the exhaustgases. The unburned HC undergoes in the DOC exothermic oxidationreactions, and is converted into water and carbon dioxide.

As explained above, it is desirable to monitor the conversionperformance of the DOC. Conventional DOC diagnostic is performed duringDPF regeneration events and based on the exothermic effect of the DOCcaused by post-injection. While performing the DOC diagnostic eventduring a regeneration event ensures that the DOC temperature will beabove the minimum temperature required for enabling the catalystmaterial, there is a risk of erroneous assessment.

This problem can be better understood from FIG. 1, which shows theeffect of ageing on a DOC, the HC conversion efficiency being plottedvs. DOC temperature. As it can be seen, an aged catalytic converter canstill have a good efficiency in the high temperature range (e.g.typically above 300° C. for many DOCs). However, the highly agedcatalyst clearly has a worse or unacceptable conversion efficiency atlower temperatures (e.g. below 250° C.).

To ensure a reliable, long term monitoring of catalytic converters,namely DOCs, the present method proposes to perform the DOC diagnosticevent at lower or moderate temperatures, where it is actually possibleto assess the ageing status of the DOC and not simply whether it isbroken or not. In the case of the ageing behaviour shown in FIG. 1, thetemperature window during which the diagnostic event can be formed inaccordance with the present method is e.g. 150 to 200° C.

Therefore, the diagnostic event is preferably performed when the engineis operated normally (normal combustion mode, e.g. lean burn) but not ina regeneration mode of DPF that causes high temperature exhaust gasesand thus brings the DOC in the high temperature range.

The DOC temperature is preferably monitored according to a multi-slicedmodel, i.e. the DOC is virtually divided into multiples slice and analgorithm is used to determine the temperature in each slice based onone or more temperature measurements of the DOC.

In the present method, the oxidation catalyst diagnostic event is thusperformed when the DOC is in the predetermined temperature window, asexplained above. This temperature window will typically be calibratedbased on experimental testing, simulation, forced ageing test, etc. fora given make and type of catalyst.

The diagnostic event involves the following test cycle. While it hasbeen checked that the DOC temperature is within the prescribed window,and that the engine is not operated in a DPF regeneration mode or otherrich combustion mode, a post-injection of a metered quantity of fuel isperformed. As it is used herein, the term post-injection designates theinjection of fuel either in-cylinder or in the exhaust piping, beforethe DOC, at a timing where no or little combustion occurs. In-cylinderpost injection may typically be executed towards the end of the powerstroke (or later). However, as compared to the post-injectiontraditionally performed in DPF regeneration mode, the amount of fuelthat is injected in the post-injection pulse is comparatively smaller.Indeed, the objective is not to actually bring the DOC in the hightemperature range, but to monitor the consequence of the post-injectionand check whether it results in the expected exotherm or not. Hence,typically a small quantity of post fuel (e.g. a few grams) is injectedto generate a temporary exotherm in the DOC.

As soon as the post-injection has been performed, the heat generated bythe DOC is accumulated (taken into account by the algorithm implementingthe test cycle). This is preferably done by measuring the temperature ofthe exhaust gases at the DOC outlet (or downstream thereof). When allthe heat stored in the DOC has been evacuated in the exhaust gases, theheat accumulation stops and is compared to the theoretical heat that thepost fuel should have produced. The conversion efficiency is thencalculated as the ratio of the accumulated heat to the theoretical heatthat the post fuel should have produced.

To evaluate the exotherm, we consider that the instantaneous total heatenergy of the exhaust gases that exit the DOC corresponds to the sum ofthe heat energy of the exhaust gases that enter the DOC and the heatenergy due to the exotherm that is linked to the injected fuel quantity,however corrected by a term representative of transients namely takinginto account the thermal inertia of the DOC. This instantaneous heatenergy balance of the DOC can be expressed as follows:

$\begin{matrix}{{\overset{.}{m} \cdot {Cp} \cdot T_{out}} = {{\overset{.}{m} \cdot {Cp} \cdot T_{in}} + {{\overset{.}{q}}_{f} \cdot H \cdot \eta} - \frac{Q_{DOC\_ stored}}{t}}} & (1)\end{matrix}$

where

T_(in): represents the exhaust gas temperature at the DOC inlet;

T_(out): represents the exhaust gas temperature at the DOC outlet;

η: is the DOC conversion efficiency;

m: represents the mass flow of the exhaust gases;

Cp: represents the heat capacity of the exhaust gases;

q_(f): represents the fuel quantity introduced in the exhaust gases bythe post fuel injection;

Q_(DOC) _(—) _(stored): is the heat energy stored in the DOC material;

H: is the fuel heating value.

When integrating it over a time period t, relation (1) is equivalent to:

$\begin{matrix}{\eta = \frac{\sum{\overset{.}{m} \cdot {Cp} \cdot \left( {T_{out} - T_{{out} - {mod}}} \right)}}{q_{f} \cdot H}} & (2)\end{matrix}$

Where T_(out-mod) represents a model DOC-out exhaust gas temperature ofan “inert” DOC, i.e. when no post-injection is performed and thustypically without exotherm. It may be noted that in this mathematicalapproach, T_(out-mod) is equivalent to a stabilized T_(in); and q_(f)·Hcorresponds to the theoretical exotherm.

The principle of this integration is illustrated in FIG. 2, wherein themeasured exhaust gas temperature T_(out) and the modeled exhaust gastemperature without post-injection T_(out-mod) are plotted vs. time. Theintegration is started with the post injection pulse and performed untilall the heat accumulated in the DOC is eliminated.

Hence in the present method, the test cycle starts with a smallpost-injection pulse/event and comprises an exotherm monitoring periodduring which the exotherm is monitored and that lasts until essentiallyall of the heat due to the post-injection and stored in the catalyst hasbeen evacuated. Preferably, the exotherm monitoring period starts withthe post-injection event and is continued after the post-injectionevent, until all of the heat has been evacuated. Typically, themonitoring of the heat in the catalyst implies measuring the exhaust gastemperature at the DOC outlet (T_(out)), and the heat is accumulated(integrated) in order to determine the quantity of heat (i.e. theexotherm) actually generated by the post-injection event. The end of themonitoring period and thus the elimination of the heat due to theexotherm can be determined by comparing the exhaust gas temperature atthe DOC outlet T_(out) to the model DOC-out exhaust gas temperatureT_(out-mod). When the difference T_(out)−T_(out-mod) drops below apredetermined threshold, it can be concluded that the heat has evacuatedand the monitoring can be stopped. In other words, the monitoring periodends when T_(out) and T_(out-mod) are substantially equal.

Alternatively, the end of the exotherm monitoring period can beindicated by the expiry of a calibrated timer. Such timer can have aduration that is known (from experimentation/testing) to be greater thanthe time required for the exotherm (caused by the known post-fuelinjection) to be evacuated.

As it is clear to those skilled in the art, the post-injection eventshown in FIG. 2 can be a continuous pulse when injected directly in theexhaust line, but will correspond to multiple additional fuel amountswhen injected in the cylinder.

For exemplary purposes, which shall not be construed as limiting, it isconsidered that the injection of a post-fuel amount of 1 to 15 g duringa period of 1 to 20 s is suitable for conducting the present diagnosticwith most types of oxidation catalyst currently in use in automotivevehicles. With such post-injection fuel, it is expected that theexotherm monitoring period will last between 1 and 10 minutes.

In order to provide an accurate result of the efficiency, the exactamount of fuel entering the DOC (q_(f)) must be known and is thusprecisely metered. For an enhanced accuracy, the post fuel injected ispreferably corrected for engine-out temperature and exhaust flow, usinge.g. a calibratable correction factor.

Also, to avoid noise due to engine-out HC amounts that are not inducedby post injection, the EGR rate (Exhaust Gas Recirculation) for carsequipped with an EGR valve permitting to recirculate part of the exhaustgases back to the intake manifold, is preferably reduced to apredetermined value during the diagnostic event. Also, any otherparameter that may affect the engine-out HC concentration, e.g. swirl inthe combustion chamber, is preferably set, for the duration of thediagnostic event, to a predetermined value that minimizes the engine-outHC.

It remains to be noted that usual oxidation catalyst technologies arebased on zeolites, which have a substantial HC storing effect at lowtemperature (typically <250° C.). To avoid unknown accumulation of HC inthe DOC prior to a diagnostic event, the start of the diagnostic eventis prevented when the DOC has been running more than a predeterminedtime period below a predetermined temperature threshold. This maytypically be the case if the engine has been running idle for a while. Adiagnostic event may subsequently be started when it is observed thatthe DOC temperature has increased above the temperature threshold,provided the other required conditions are met.

Besides, for a reliable DOC performance monitoring, the diagnostic eventis preferably cancelled if the DOC temperatures goes out of thediagnostic temperature window. And in particular if the DOC temperaturegoes out of the diagnostic temperature window during the performance ofthe post-injection event of the test cycle. In this respect it may benoted that since the post-injection is relatively short, the likelihoodthat the DOC temperature exits the diagnostic temperature window is low.

It shall further be appreciated that due to the fact that only a shortpost-injection event (and a thus a small fuel amount as compared to thefuel amounts typically post-injected in regeneration mode) is used inthe test cycle, the generated exotherm will typically not cause anoverheating of the DOC that will bring its temperature outside thediagnostic window.

Referring now more specifically to the selection of the diagnostictemperature window, the idea is to operate the diagnostic in a DOCtemperature window where an aged, namely a highly aged catalyst cannotbe assessed as less aged catalyst. In practice this implies determiningthe limits of the diagnostic temperature window based on a highly agedcatalyst that serves as a reference (reference oxidation catalyst). Theupper limit of the window should be selected to correspond to aconversion efficiency below 100% of the reference catalyst, preferablybelow 90%. In the example of FIG. 1, this means a upper limit for thetime window of 200° C. The lower limit may be, as already mentioned 150°C., which is a temperature at which conversion is enabled, althoughquite low. An additional criterion for selecting the lower limit may bethat the temperature is sufficient for a functional oxidation catalyst,e.g. as the mildly aged catalyst of FIG. 1, to reach the light-offpoint, i.e. a conversion efficiency of 50% (which is the case here at150° C.).

In an alternative, preferred embodiment, the diagnostic temperaturewindow may be selected to correspond to a conversion efficiency rangingfrom 40 to 60% for the reference, highly aged catalyst. Applying this toFIG. 1, it can be deduced from the curve of the highly aged catalyst(reference) that the corresponding diagnostic temperature window is 170°C. to 185° C. As it can be seen, this temperature range corresponds to asteep transition zone of the conversion efficiency of this reference(highly aged) catalyst. It is notably remote from the conversionefficiency curve of the mildly aged catalyst.

Still with reference to the example of FIG. 1, if the determinedconversion efficiency is below 40% in this predetermined diagnostictemperature window (170-185° C.), then it is concluded that theoxidation catalyst needs to be replaced and an alert signal isgenerated.

The present method can be easily implemented in the ECU of an internalcombustion engine and does not require additional, specific equipment(for most engines/vehicles).

1. A method for monitoring an oxidation catalyst in an exhaust line ofan internal combustion engine, wherein a catalyst diagnostic eventcomprises a test cycle during which a conversion capability of saidoxidation catalyst is determined based on an exotherm generated bypost-injection of fuel, wherein said diagnostic event may only beinitiated when the temperature of said oxidation catalyst lies within apredetermined temperature range, and wherein a post-injection event andan exotherm monitoring period that lasts, after said post-injectionevent, until essentially all of the heat stored in said oxidationcatalyst has been evacuated.
 2. A method according to claim 1, whereinthe exotherm is evaluated by measuring the temperature of exhaust gasesexiting said oxidation catalyst during said exotherm monitoring period.3. A method according to claim 2, wherein said oxidation catalyst has anoutlet, and wherein said exotherm monitoring period ends when theexhaust gas temperature at the catalyst outlet (T_(out)) and a modelcatalyst-out exhaust gas temperature T_(out-mod) are substantiallyequal.
 4. A method according to claim 2, wherein said exothermmonitoring period ends when a difference between the exhaust gastemperature at the catalyst outlet (T_(out)) and a model catalyst-outexhaust gas temperature T_(out-mod) drops below a predeterminedthreshold.
 5. A method according to claim 2, wherein said exothermmonitoring period ends with the expiry of a calibrated timer.
 6. Amethod according to claim 1, wherein said predetermined temperaturerange is selected to correspond to a transition zone of the conversionefficiency for a reference oxidation catalyst with given ageing.
 7. Amethod according to claim 1, wherein said predetermined temperaturerange has an upper temperature limit selected to correspond to aconversion efficiency of no more than 90% for an aged, referenceoxidation catalyst.
 8. A method according to claim 1, wherein saidpredetermined temperature range has lower and upper temperature limitsthat are selected to correspond to a conversion efficiency of about 40to 80%, respectively, for an aged, reference oxidation catalyst.
 9. Amethod according to claim 1, wherein the lower temperature of saidpredetermined temperature range is selected to at least correspond to apredetermined conversion efficiency of a functional, reference oxidationcatalyst.
 10. A method according to claim 1, wherein said diagnosticevent is performed when the engine is not operated in a DPF regenerationmode.
 11. A method according to claim 1, wherein said diagnostic eventis cancelled in case the temperature of said oxidation catalyst exitssaid predetermined temperature range.
 12. A method according to claim 1,wherein said diagnostic event is cancelled in case the temperature ofsaid oxidation catalyst exits said predetermined temperature rangeduring the performance of said post-injection event.
 13. A methodaccording to claim 1, wherein the oxidation catalyst temperature ismonitored by means of a multi-slice model, and said diagnostic event mayonly be triggered and/or performed when the respective temperatures ofall of said slices lie within said predetermined temperature range. 14.A method according to claim 1, wherein during said diagnostic cycleengine parameters are set to minimize the amount of HC in the engine-outexhaust gases, except for said post injection event.
 15. A methodaccording to claim 1, wherein an alert signal is triggered when theconversion efficiency drops below a conversion efficiency alertthreshold.
 16. A method according to claim 1, wherein said predeterminedtemperature range has lower and upper temperature limits that areselected to correspond to a conversion efficiency of about 40 to 60%,respectively, for an aged, reference oxidation catalyst.
 17. A methodaccording to claim 9, wherein said predetermined conversion efficiencyis of at least 50%.